text: The four-factor formula, also known as Fermi's four factor formula is used in nuclear engineering to determine the multiplication of a nuclear chain reaction in an infinite medium.
  The symbols are defined as:
  *,  and  are the average number of neutrons produced per fission in the medium (2.43 for uranium-235).
  * and  are the microscopic fission and absorption thermal cross sections for fuel, respectively.
  * and  are the macroscopic absorption thermal cross sections in fuel and in total, respectively.
  * is the number density of atoms of a specific nuclide.
  * is the resonance integral for absorption of a specific nuclide.
  * is the average lethargy gain per scattering event.
  **Lethargy is defined as decrease in neutron energy.
  * (fast utilization) is the probability that a fast neutron is absorbed in fuel.
  * is the probability that a fast neutron absorption in fuel causes fission.
  * is the probability that a thermal neutron absorption in fuel causes fission.
  * is the thermal non-leakage probability